In recent years, a hybrid vehicle comprising an engine and a motor generator has been proposed for the purpose of saving a fuel for driving the engine, reducing noise resulting from a rotation of the engine, and reducing emission resulting from combustion of the fuel. In the hybrid vehicle, the engine and the motor generator are controlled on the basis of a running condition of the vehicle when the vehicle is running.
Specifically, the hybrid vehicle can be run by driving the engine in a revolution region where combustion efficiency thereof is preferable, and also by using the motor generator as a motor while halting the engine in a revolution region where the combustion efficiency of the engine is degraded. One example of a control system for a drive system thus having an engine and a motor generator is disclosed in Japanese Patent Laid-Open No. 2003-127681. According to the system disclosed in Japanese Patent Laid-Open No. 2003-127681, an output torque of the engine is transmitted to a wheel via a planetary gear mechanism, a transmission, and a differential device. The planetary gear mechanism comprises a sun gear, a ring gear and a carrier, and a crankshaft of the engine is connected with the carrier. Also, a first motor/generator is provided and a rotor thereof is connected with the sun gear. The ring gear is connected with a propeller shaft, and the propeller shaft is connected with the transmission. On the other hand, a second motor/generator is provided and a rotor thereof is connected with the propeller shaft.
Japanese Patent Laid-Open No. 2003-127681 describes that the halted engine can be started by carrying out a cranking by the torque of the first motor/generator, and then, fuel is fed to the engine and combusted to rotate the engine autonomously. However, a resonance (i.e., vibration) or noise may be amplified in a power transmission system including the first motor/generator in case of transmitting the torque of the first motor/generator to the engine.